Image capture systems are known to comprise an optical system, a digital image sensor and an autofocus module.
Optical system refers to an optical system comprising one or several elements in particular including lenses, mirrors and/or diffractive elements.
The autofocus module comprises an automatic adjustment function for adjusting the system's focus so as to obtain a sharp image.
The focusing operation consists in converging the light rays exiting the optical system at the image sensor. The term converge involves minimizing the size of the point spread function obtained at the image sensor.
This focus is generally determined by successive iterations, by browsing through the different configurations accessible with the autofocus module and by measuring the sharpness of each of these configurations, for example for the green color component when this is a color image.
It should be noted that in this document, the terms “color component” and “color” correspond to the same notion.
The use of the autofocus module however presents two main disadvantages: the time taken to determine the optimal configuration can be long (this time is referred to as autofocus latency) and the amount of electricity consumed in particular to produce movements is high.
FIG. 5 demonstrates the electrical consumption of an image capture system with an autofocus module from the prior art: the curve L1 between the points J1 and K1 represents the electrical intensity values according to the defocus values in the case of a system focusing between an object placed at a focus distance of infinity and a close object. The point J1 corresponds to zero defocus (object at a distance of infinity) and the point K1 corresponds to maximum defocus (close object).
This figure is provided for illustration purposes with the level of electrical consumption required for defocus capable of varying according to the type of autofocus module used.
Moreover, this is why the autofocus module is not activated in most camera phones (it is set for an object at a distance of infinity) when the user uses the camera phone in preview mode (i.e. when the image is only captured at a low resolution (the resolution refers to the number of pixels contained in the image): for example 320 pixels*400 pixels at most, before the true image capture taken by the user, at a higher resolution). The autofocus mode is only activated at the time when the picture is taken by the user. The latter therefore a priori does not know whether the image taken at this instance will be sharp or blurred.